Preheat your oven to 180°c (160°c for fan-assisted ovens, Gas Mark 4 or 3500°F) and grease a 20-22cm loose-bottom pie tin before dusting it with a little excess flour. Set aside.
Roughly chop your apples and set aside. In a small bowl, mix the sugar, cinnamon & cornflour and set aside.
Take your chilled pastry from the fridge and remove about ⅓ of it, setting it aside for the top of the pie. Roll the remaining pastry on a well-floured work surface until it is big enough to line your pie tin.
Gently lift the pastry into your prepared pie tin, pressing it into the edges and use any excess pastry to patch up any breaks or holes.
Add your apples to your pastry-lined pie tin, ensuring they are evenly distributed. Sprinkle the sugar mixture on top of the apples.
Roll your remaining pastry until it is large enough to cover your pie. Wet the edges of your pie with a little water before transferring the pastry lid to the top, pressing the edges together to seal the pie. Again, patch up any holes or breaks with excess pastry.
Trim the edges of the pastry before making a few holes in the top to allow the steam to escape while baking.
Bake in your pre-heated oven for around 40-50 minutes, until golden on top.
Remove from the oven and sprinkle with some caster sugar whilst still hot. Allow to cool in the tin for a while before slicing and serving while still a little warm.